1. Field of the Invention
The present invention relates to an optical scanning apparatus, and particularly to an optical scanning apparatus usable in an apparatus with an electrophotographic process, such as a laser beam printer and a digital copying or duplicating machine, in which a light beam is scanningly deflected by a deflector onto a surface to be scanned through an f.theta. lens to record picture information on the scanned surface.
2. Related Background Art
Conventionally, a light beam emitted from a light source means is modulated according to a modulation signal in an optical scanning apparatus such as a laser beam printer (LBP). The modulated light beam is periodically deflected by an optical deflector such as a rotating polygon mirror, the deflected beam is condensed into a spot by an image-forming optical system having f.theta. characteristics and a photosensitive recording medium is scanned with the condensed spot to record picture information thereon.
FIG. 1 schematically shows a prior art optical scanning system, and this figure is a cross-sectional view taken along or with respect to a main-scanning direction. In FIG. 1, a diverging light beam emitted from a light source means 1 is collimated by a collimator lens 12, and the collimated light beam is restricted or limited by a stop 3 to be incident on a cylindrical lens 4. Along the main-scan direction, the collimated light beam incident on the cylindrical lens 4 passes therethrough as it is, i.e. without any change. Along the sub-scan direction which is perpendicular to the main-scan direction, the collimated light beam is converged by the cylindrical lens 4 and is imaged as a substantially linear image on a reflection surface 5a of an optical deflector which consists of a polygon mirror.
The light beam is reflectively deflected by the reflection surface 5a of the deflector 5 and is directed to a surface 7 to be scanned through an image-forming optical system which has f.theta. characteristics and is disposed so that the optical axis of its lens surface is coincident with the optical axis of the optical scanning system. The optical system 16 consists of lenses 16a and 16b made of glass. The scanned surface 7 is scanned with the imaged light beam by rotating the optical deflector 5 in a direction of an arrow A.
In the prior art optical scanning apparatus, the scanning lens system 16 consisting of the two lenses 16a and 16b is used as the image-forming optical system for condensing the light beam deflected by the deflector 5 onto the scanned surface 7.
In recent years, in order to reduce the size of the optical scanning apparatus and the manufacturing cost thereof, it has been considered to make up the above-discussed scanning lens system of a single lens of synthetic resin. While the synthetic resin lens is advantageous in reducing the cost and in other points, it is difficult to make accurate the surface precision of the lens when its thickness is large.
Therefore, it is desired to thin the thickness of the lens. If the lens is thinned, however, the power of the lens is lessened to the point where the power may be insufficient to image the light beam on the surface to be scanned. Hence, it is desirable to cause a converging light beam, and not a collimated beam, to be incident on the lens for compensating for insufficient lens power. To satisfy such a requirement, it is considered effective to construct the optical scanning apparatus with a light source, a first optical system for converting a light beam from the light source to a convergent light beam, a deflector for deflecting a light beam from the first optical system, and a second optical system for imaging the convergent light beam deflected by the deflector into a spot on a scanned surface.
When the convergent light beam is caused to enter the second optical system (scanning lens), the rotational center of the deflector is determined so that a zero-percent light beam (i.e., a light beam deflected by the deflector when a center of a recording area is scanned therewith) is coincident with the optical axis of the scanning lens.
FIG. 2 shows the reason this desired relation. If the zero-percent light beam were to enter tile scanning lens with a hight H relative to the optical axis of the scanning lens, the zero-percent light beam is not imaged at the center of the recording area, but rather deviated from the center by distance h. This deviation results because the zero-percent light beam is a convergent light beam, and therefore the image of the beam is formed closer in than the focal length f of the scanning lens.
However, despite the desirability of this relation, when the rotational center of the deflector is arranged so that the zero-percent light beam is coincident with the optical axis of the scanning lens, light beams that scan peripheries of the recording area and deflected with the same angle .theta. relative to the zero-percent light beam are not symmetrical with each other with respect to the zero-percent light beam, as shown in FIG. 3. FIG. 3 illustrates such situation in the main-scan cross section. Because of the non-symmetry, distances from tile center of the recording area to both opposite peripheries are different from each other, that is, in FIG. 3, E&lt;E'.
As a result, when an f.theta. lens designed to be symmetrical with respect to the optical axis in a main-scan cross section is used as a scanning lens, the f.theta. characteristic considered with the image-formed position of an on-axis or zero-percent light beam as a center will not be symmetrical with respect to a symmetrical axis of the f.theta. lens system in the main-scan cross section. In particular, since the f.theta. characteristic exhibits reverse characteristics between picture image record starting side and its opposite side, such asymmetry becomes a problem.